Recently McCauley and Racker (8) have demonstrated that MAO purified from bovine brain exists in two immunologically and catalytically distinct forms MAO A and MAO B. These two enzymes have catalytic properties that are similar to activities measured by others in intact mitochondria of a number of species including the laboratory rat (22). In general terms, the objective of this proposal is to prepare monospecific antibodies to each of these enzyme forms and to use these antibodies to study the structural differences between MAO A and MAO B, the hepatic biosynthesis and the retional distribution of these enzymes in brain tissues. These experiments will be performed on the laboratory rat.